The use of photoinitiator systems to initiate photopolymerization is well known. When irradiated by actinic radiation, the photoinitiator system generates free radicals which initiate polymerization of the monomer or monomers. The photoinitiator system may be a single compound which absorbs actinic radiation and forms the initiating radicals or it may consist of several different materials which undergo a complex series of reactions to produce radicals. Added components, which do not absorb actinic radiation, but which increase the efficiency of the photoinitiator system, are known as coinitiators.
Borate anion coinitiators have recently been disclosed. Gottschalk et al., U.S. Pat. Nos. 4,772,530 and 4,772,541, disclose photopolymerizable compositions containing a cationic dye-borate anion complex in which the cationic dye-borate anion complex is capable of absorbing actinic radiation and producing free radicals. Cationic methine, polymethine, triarylmethane, indoline, thiazine, xanthene, oxazine, and acridine were disclosed. Triaryl alkyl borate anions were the preferred borate coinitiators.
Yamaguchi et al., U.S. Pat. No. 4,902,604, discloses photopolymerizable compositions containing a salt formed by an organic cationic dye compound and a borate anion. In these salts the cationic dye compound comprised an azulene ring having a nitrogen atom or a chalcogen atom in the 2- or 4-position.
Koike et al., Federal Republic of Germany patent application 3,822,921 equivalent to U.S. Pat. No. 4,950,581, discloses photopolymerizable compositions containing an organic dye and a triaryl butyl borate anion coinitiator. In these systems the dye did not contain a counter anion. Merocyanine type dyes, coumarin type dyes, and xanthene and thioxanthene dyes were disclosed.
Despite the many improvements made in photoinitiator systems, a need exists for photopolymerizable compositions with increased photospeed. With increased photospeed, shorter irradiation times are possible. The time and effort necessary to prepare an image is reduced, and the capacity of existing equipment is increased. Since exposure times are reduced, increased photospeed frequently leads to improved image quality as well.